Emergency oxygen supply systems are commonly installed on aircraft for the purpose of supplying oxygen to passengers upon loss of cabin pressure at altitudes above about 12,000 feet. Such systems typically include a face mask adapted to fit over the mouth and nose which is released from an overhead storage compartment when needed. Supplemental oxygen delivered by the mask increases the level of blood oxygen saturation in the mask user beyond what would be experienced if ambient air were breathed at the prevailing cabin pressure altitude condition. The flow of oxygen provided thereby is calculated to be sufficient to sustain all passengers until cabin pressure is reestablished or until a lower, safer altitude can be reached.
Each such face mask has a reservoir bag attached thereto into which a constant flow of oxygen is directed upon deployment of the system and upon activation of the individual face mask via a pull cord. The oxygen is supplied continuously at a rate that is calculated to accommodate a worst case scenario, namely to satisfy the need of a passenger with a significantly larger than average tidal volume who is breathing at a faster than average respiration rate when cabin pressure is lost at maximum cruising altitude. A total of three valves that are associated with the mask serve to coordinate flows between the bag and the mask, and between the mask and the surroundings. An inhalation valve serves to confine the oxygen flowing into the bag to the bag while the passenger is exhaling as well as during the post-expiratory pause and at all times also prevents any flow from the mask into the bag. When the passenger inhales, the inhalation valve opens to allow for the inhalation of the oxygen that has accumulated in the bag. Upon depletion of the accumulated oxygen, the dilution valve opens to allow cabin air to be drawn into the mask. The continuing flow of oxygen into the bag and through the open inhalation valve into the mask is thereby diluted by the cabin air that is inhaled during the balance of the inhalation phase. During exhalation, the exhalation valve opens to allow a free flow from the mask into the surroundings while the inhalation valve closes to prevent flow from the mask back into the bag. All three valves remain closed during the post-expiratory pause while oxygen continues to flow into the reservoir bag.
Inefficiencies in an emergency oxygen supply system can require the oxygen storage or oxygen generation means to be larger and therefore weigh more than necessary which of course has an adverse impact on the payload capacity and fuel consumption of the aircraft. Enhancing the efficiency of such a system either in terms of the generation, storage, distribution or consumption of oxygen could therefore yield a weight savings. Conversely, an enhancement of a system's efficiency without a commensurate downsizing would impart a larger margin of safety in the system's operation. It is therefore highly desirable to enhance the efficiency of an emergency oxygen supply system in any way possible.